Soliton transmission systems can potentially provide exceedingly high information transmission capacity over long distances. In ultra-long distance systems such as transcontinental and transoceanic systems, optical amplifiers periodically boost the power of propagating information-bearing soliton pulses sufficiently high to compensate for losses experienced in the fiber transmission medium. Unfortunately for ultra-long distance systems, however, the maximum information bit rate for a single channel is set by the amount of jitter in pulse arrival times generated by two different effects. One is the Gordon-Haus effect and the other is an acoustic interaction effect.
The Gordon-Haus effect is occasioned by the interaction of soliton pulses with amplifier spontaneous emission noise present along the transmission medium. J. P. Gordon et al. describe this effect in Optic Letters, Vol. 11, No. 10, pp. 665-7 (1986). Amplifier spontaneous emission noise alters both the amplitude and carrier or channel frequency of the soliton pulses at random resulting in a jitter in pulse arrival times. Pulse jitter can cause a soliton pulse to walk off into the time interval reserved for a neighboring soliton pulse. The result, often known as intersymbol interference, is an error in the received information.
Recently, M. Nakazawa et al. suggested in Electronics Letters, Vol. 27, p. 1270 (1991) that active electronic devices such as modulators to provide time domain filtering could be used to eliminate soliton pulse arrival time jitter. This technique is not only costly, complex, and difficult to implement but it also suffers the same incompatibility with wavelength-division-multiplexing experienced by electronic regeneration of optical signals.
A simpler alternative for reducing jitter from the Gordon-Haus effect was described in copending, allowed U.S. patent application Ser. No. 07/744,615 (Hasegawa et al. Case 8-3-19) and disclosed in articles by Y. Kodama et al. in Optics Letters, Vol. 17, No. 1, pp. 31-3 (1992) and by A. Mecozzi et al. in Optics Letters, Vol. 16, No. 23, pp. 1841-3 (1991). These references propose the use of linear narrow-band filters ("frequency guiding filters") spaced at predetermined intervals along the transmission fiber. Each filter, in essence, shapes the frequency dependent gain characteristic of the corresponding amplifier. The linear filters are effectively identical within manufacturing tolerances in that each filter exhibits a center frequency substantially equal to the soliton center frequency. However, the introduction of filters causes additional soliton pulse energy loss which, in turn, must be offset by higher gain from the optical amplifiers. This higher gain, however, results in an exponential increase with distance of those spectral components of the noise at or near the filter response peak. As a result, the maximum useable filter strength is limited as is the realizable jitter reduction. A recent experiment using the frequency guiding filters was reported by L. Mollenauer et al. in Electronics Letters, Vol. 28, p. 792 (1992) for a 10,000 km soliton transmission system in which the filter achieving the lowest bit error rate caused a 50% reduction in the standard deviation of the timing jitter.